The invention relates to a method for reducing the emission of the waste gas pollutants dioxin and/or furan contained especially in a hot partial gas extraction with which a respective pollutant-containing partial stream from the rotary kiln waste gas of a system for manufacturing cement clinkers is extracted, cooled in a mixing chamber and dedusted in an electrostatic dust collector, and the cement clinker from the rotary kiln is cooled in a connected cooler and an exhaust air stream (tertiary air) of the clinker cooler is used separately or with the rotary kiln waste gas for precalcining the cement raw meal.
In the cement technology, it is known that many materials used for manufacturing cement clinker contain minor constituents, such as alkali compounds, sulfur, chlorine, heavy metals, etc. which partially evaporate in the area of the sintering zone, recondense in the preheater area and thus build up circuits which can negatively affect the quality of the cement clinker and interfere considerably with the combustion process itself. In order to prevent an alkali chloride circulation in a cement clinker combustion system, for example, and reduce the alkali chloride content in the cement clinker, it is known in the art to branch off and extract a part of the rotary kiln waste gas containing pollutants via a so-called partial gas extraction in the lower area of the waste gas ascension pipe between the rotary kiln and the cyclone suspension-type heat exchanger system, and at a temperature level at which the volatile pollutants are not yet condensed or only partially condensed. If the extracted partial gas stream is high in alkali chloride, the partial gas stream is cooled and then dedusted in its own electrostatic dust collector. If the alkali chlorides are not fully condensed, and if the partial gas extraction stream still contains unacceptable levels of gaseous/vaporous pollutants, the partial gas extraction would have to be discarded and purified in its own waste gas purification system (article entitled “Problems of electrostatic dust removal from waste gases high in alkali chloride of partial gas extractions in heat exchanger-type dry rotary kilns” in the trade magazine Zement-Kalk-Gips [cement-lime-gypsum] no. 5/1978, pages 236 to 238).
However, in addition to the formation of alkali chlorides, the formation of dioxins and/or furans increasingly represents a problem in manufacturing cement clinker. The formation of the waste gas pollutants dioxin and furan in the cement clinker manufacturing process is conceivable in three different ways:
Caused by raw materials. Because of the organically bound carbon and the chlorides existing in limestone it is possible for dioxin to develop in the upper stages of the cyclone suspension-type heat exchanger system of the raw meal preheater. When the formed dioxins are not particle-bound and therefore are not incorporated into the cement clinker, they are able to escape to the outside with the waste gas. The countermeasures are limited, because a certain degree of TOC (total organic carbon) is naturally present in the cement raw meal and therefore, it cannot be prevented.
Caused by combustion fuel. The formation of dioxin is not expected when chlorinated combustion fuel, that is, fuel containing chlorine compounds, such as secondary fuel or waste fuel, is burned in the burner of the rotary kiln because of the high combustion temperatures. However, the probability of dioxin formation increases when burning chlorinated fuel in secondary combustion, that is, in the precalcinator preceding the rotary kiln in relation to the flow of materials because there, the temperature level of the fuel combustion is lower, and frequently no more than approx. 850° C. The third possibility for forming dioxin is caused by special dust removal systems, especially for the bypass gases or partial gas extractions in cement clinker production lines, by means of electrostatic dust collectors (E-filters). Arc-overs may be caused in such E-filters because of the high electric voltage. It was found that the combination of the prevailing temperatures and the high chloride content support the formation of dioxin. Said source of dioxin formation could be bypassed in that fabric filters are preferably installed for dust removal in new or rebuilt cement clinker production lines. There are many cases, however, where the effectively operating electrostatic dust collectors should not be avoided.
In EP-B-0 534 225, it was proposed to cool and dedust a bypass gas or a respective partial gas extraction containing the pollutants SO2 and NOx and then recirculate this gas in the clinker cooler and via the secondary air in the rotary kiln so as to bind the SO2 in the raw meal preheater to the lime component of the cement raw meal and reduce the NOx in the De-Nox reduction zone of the rotary kiln waste gas ascending pipe of the precalcinator. However, the publication does not address reducing the emission of the waste gas pollutants dioxin and/or furan, such as may occur under certain circumstances in cement clinker production lines.